The present invention relates to a control system for an electromagnetic clutch for an automatic transmission of a motor vehicle.
An automobile provided with a continuously variable belt-drive transmission with an electromagnetic clutch is disclosed in EP-A 151038. The electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes as steady state modes, such as a starting mode of a vehicle, reverse excitation mode, drag mode, mode of lock-up engagement. One of the modes is selected in accordance with a position of a selector lever and driving conditions to control the electromagnetic clutch.
Other prior art discloses control systems which provide transient state modes for various transient states. The transient states include states at the changing of ranges, for example changes from a neutral range (N-range) or parking range (P-range) to drive range (D-range), high engine speed drive range (Ds-range) or reverse range (R-range), from the D-range to Ds-range, and at the changing of modes, for example changes from the starting mode to the lock-up engage mode. Further, the transient state mode includes operations such as releasing and depressing of an accelerator pedal in starting or lock-up engage mode. In the transient state mode, clutch torque is temporarily decreased to cause the clutch to slip so as to prevent a shock at the changing operation. The clutch torque is gradually raised thereafter. In another device, the clutch torque is varied with time in accordance with a particular characteristic.
In order to appropriately control clutch slipping characteristic, controlling of the clutch engagement ratio is proposed. In the control, desired clutch engagement ratio Eo is set and actual clutch engagement rate E is obtained. The clutch torque is increased or decreased in proportion to the difference between both the desired ratio and the actual ratio (Eo-E) to converge the actual ratio to the desired ratio. However, in the transient state mode, rapid response to variations of engine torque and torque-load is required. Since hunting or overshooting of the engagement ratio control causes the driveability of the vehicle to reduce, it is important to improve the convergency.